Summary:
The Corani Project’s mineralization is a low to intermediate sulfidation epithermal system with silver, lead, and zinc deposits hosted in stock works, breccia veins, and fractures. The gold zone to the south is a low sulfidation epithermal gold occurrence in association with silica. The antimony zone is comprised of stibnite-pyrite veins with silica. There is also sulfide mineralization in the sediments that are essentially barren of silver and lead.
The above combinations are indicative of the epithermal mineralization that is sometimes associated with distal zoning around a porphyritic intrusion.
Mineralization at the Corani Project occurs in three distinct and separate zones: Corani Main, Corani Minas, and Corani Este, each differing slightly in character with regard to both alteration and mineral assemblages. In general, mineralization in outcrop throughout the Corani Project is associated with iron and manganese oxides, barite, and silica. Silicification is both pervasive and structurally controlled along veins. In drill core, the mineralization occurs in typical low to intermediate sulfidation Ag-Pb-Zn mineral assemblages.
The most abundant silver-bearing mineral is fine-grained argentian tetrahedrite or freibergite (Hazen Research Inc., 2006; Gunning, 2007). Other minor silver minerals present include acanthite and the lead-silver sulfosalts, adorite and diaphorite. Other sulfide minerals include pyrite-marcasite, boulangerite, sphalerite, and galena. Boulangerite and galena do not appear to be significant hosts for the silver. Sphalerite, mainly high Fe type, overlaps the silver mineralization but can be more areally extensive, particularly at Corani Minas, where sphalerite may extend 10 to 100 m beneath and lateral to the silver-bearing minerals. Lead also occurs as plumbogummite, a lead-aluminum phosphate. Lead mineral speciation is dependent on pH, and the plumbogummite is believed to be secondary in origin, forming as a result of the remobilization of lead in the presence of phosphate in a very acidic environment with abundant aluminum.
Corani Main
The primary mineralized vein breccia in the Corani Main area can be traced for 800 m and undulates with strikes and dips varying between S50°E, 55°W and S20°W, 40°W, with steepening dips to the north. Vein breccias locally attain widths of greater than 10 m. To the north, strike and dip change, which indicates a plunging, dilatant structure that attains widths of 80 m, with near vertical quartz veins surrounded by stockwork systems in adjacent wall rock. The change in strike suggests that an overall sinistral (left-lateral) strike-slip component affects the veining, causing the mineralization to blow-out, or widen and intensify, to the north (Nelson, 2006). Breccias and stockworks are characterized by chalcedonic, cockscomb, crystalline, hyaline, and amethystine quartz, with hematite-jarosite-goethite and barite stringers resulting in a highly banded texture. Manganese oxides are generally sparse but locally abundant. Pyrite, dark sphalerite, and freibergite are present (Petersen, 2005; Espinoza, 2006). A post-mineral transverse fault, striking N70°E and showing dextral movement, separates the Corani Main and Corani Minas mineralized areas. The fault deforms the veins on either side; however, post-mineral displacement appears minimal, despite the fault’s appearance as a major lineament.
Corani Minas
Corani Minas is structurally complex and characterized by a large area of small, crested ridges formed by breccias, silicification, and quartz vein ribs. Veins, 0.1 to 2.0 m wide, are composed of banded, chalcedonic, and hyaline quartz, barite, hematite, jarosite, goethite, pyrite, and proustite-pyrargyrite. The veins generally strike north 20°-60° west with 50° to 80° dips to both the west and east (Desrochers, 2005; Nelson, 2006; Prado, 2008), although almost any dip angle can be observed (Nelson, 2006). Hanging-wall breccias are composed mainly of subrounded to angular clasts with void-filling barite crystals in a siliceous matrix, whereas stockwork veining is mainly within the footwall. According to Nelson, orientations and textures, such as the absence of slickensides, indicate that veining was extensional and that block rotations tended to occur surrounding an east-west axis.
Corani Este
Corani Este is distinct in that mineralization is controlled by a single listric fault that does not crop out due to post-mineral tuff cover. Silicified breccias and stockwork veining are formed in the hanging wall of the main vein and crop out as silica-rich ribs along a north-south strike. Dips are difficult to determine in outcrop and drill core due to the structural complexity and hydrothermal alteration; however, conjugate vein sets, occasionally north-south striking and dipping both east and west, are observed. Importantly, a small breccia pipe occurs in Corani Este containing high-grade silver values (=300 g/t silver). Mineralization within the pipe occurs in hydrothermal breccias with a dark-gray, sulfide matrix and dark-purple, jasperoid vein breccias.